Glucocorticoid receptors in cytosol preparations are rendered unable to bind steroid through the action of endogenous enzymes that appear to be phosphatases (Nielsen et al, J. Biol. Chem. 252:7568, 1977). We are now able to partially reactivate receptors in cytosols from mouse fibroblasts and rat thymic lymphocytes (Sando, LaForest and Pratt J. Biol. Chem. in press and Sando, Stratford, Hammond and Pratt J. Biol. Chem. in press). This activation of the receptor to the steroid binding state is temperature dependent and requires both ATP and magnesium. In the proposed work we will focus on trying to obtain receptor activation with partially purified components in order to provide a more detailed picture of the biochemical mechanism of activation. A particular goal is to try to obtain evidence for transfer of radiolabeled phosphate from ATP to a steroid-binding protein.